Liquid-fueled internal combustion engines have been used to produce power and drive machines for over a century. From the beginning, internal combustion engines have undergone many improvements to become more efficient, more powerful, and/or less polluting. To assist with these improvements, fuel properties and quality have also improved, and alternative fuels such as methanol and other alcohol-based fuels have also been considered to help with reducing harmful emissions. However, compared to such liquid fuels, an equivalent amount of a combustible gaseous fuel, such as methane, propane, butane, hydrogen, natural gas, and blends of such fuels, with equivalence measured on an energy basis, can be combusted to produce the same power while producing less harmful emissions in the form of particulates and greenhouse gases.
However, a problem with replacing liquid fuel with some of these gaseous fuels in a conventional internal combustion engine has been that they typically do not ignite as readily or at the same rate as liquid fuels. There are also many other differences that result when a gaseous fuel is substituted for a liquid fuel. For example, the combustion strategy may be different to account for longer ignition delays associated with a gaseous fuel, or a longer time may be required to inject a gaseous fuel into the engine. In addition, the fuel supply system and the manner of introducing the fuel into the engine typically require equipment specialized for handling gaseous fuels. Furthermore, the selected combustion strategy can dictate a different geometry for the combustion chamber. Accordingly, a design suitable for a liquid-fueled engine may not be suitable for a gaseous-fueled engine without considerable modifications, which can influence commercial viability.
Gaseous-fueled engines currently used in commercial vehicles operate using the Otto cycle with homogeneous mixture formation, spark ignition, and throttle control, and these engines are predominantly derived from modified diesel-cycle engines, because of the durability, power and torque required for commercial vehicles. For example, the mixture forming process, modified from that of diesel-cycle engines, as well as the use of spark ignition, are aspects that require respective modifications of the intake system and the cylinder head. The modified combustion process also necessitates a modified combustion chamber recess in the piston. Engine manufacturers usually make efforts to keep the number of engine components to be modified for gaseous fuel operation as low as possible. This is an attempt to limit the additional manufacturing costs for adapting engines to use gaseous fuel, if possible, while maintaining the durability and long service life that operators of conventionally-fueled engines have become accustomed to for their commercial vehicles.
For gaseous-fueled internal combustion engines, one of the predominant combustion processes operates with stoichiometric fuel-air mixtures in combination with a three-way catalytic converter to reduce emissions. Initially demand for gaseous-fueled engines in commercial vehicles was based on the desire for low-emission characteristics, with efficiency and fuel consumption characteristics being secondary considerations. The admixture of gaseous fuel typically takes place through a gaseous fuel mixer, arranged in the center of the intake system, with electronically controlled gaseous fuel supply. More recent gaseous fuel systems have switched to multipoint injection in front of the intake valve of each cylinder, to improve equal distribution of the fuel and to maintain a stoichiometric mixture composition during non-stationary engine operation. In order to maintain the stoichiometric (λ=1) fuel-air mixture, a ‘closed-loop’ air/fuel ratio control known from gasoline engines can be employed. The compression ratio is generally limited to values between 11:1 and 11.5:1 to ensure a sufficient safety margin against knocking.
The performance that can be achieved by non-supercharged engines with stoichiometric control is at least 5% below that of naturally aspirated liquid-fueled diesel-cycle engines, caused by the decreased air volume drawn in by the engine, which results from the addition of the gaseous fuel into the intake pipe. Compared to today's supercharged liquid-fueled diesel-cycle engines, gaseous-fueled Otto cycle engines produce up to 15% less power, taking into account the effect of the higher thermal loads and the knock limit associated with Otto cycle engines. This loss in power already takes into account that the use of exhaust gas recirculation with EGR rates of up to 15% can reduce the thermal load. The practical way to compensate for the lower performance of Otto cycle engines is to increase the displacement.
The fuel economy of stoichiometrically-controlled gaseous-fueled engines is characterized by an energy consumption that is 15 to 20% higher in stationary 13 mode tests than that of comparable diesel engines. When operating frequently under low load, as is typical for buses operating in cities, the throttle control has been found to be responsible for an increase in fuel consumption of above 40%.
The disadvantages with respect to power and fuel economy of stoichiometrically-controlled gaseous-fueled engines, in comparison to today's liquid-fueled diesel cycle engines, can be significantly reduced by employing lean-mix engine concepts. Mixture formation usually takes place downstream of the turbo charger in an electronically controlled fuel-air mixer centrally located in the intake system. For compression ratios between 11:1 and 11.5:1, the lean-mix engine as a rule possesses a combustion chamber geometry similar to those of stoichiometrically-controlled engines. Since leaner natural gas fuel-air mixtures lead to a strongly decreasing rate of combustion, a suitable adjustment of, for example, the squish flow is necessary or desirable to counteract a prolonged combustion process with accordingly higher hydrocarbon emissions. Air ratios achievable by today's lean-mix engines are not greater than λ=1.5 for high engine loads, making higher rates of combustion possible. At low engine loads, the combustion temperature is lower and the ability to operate on a lean mixture is thus limited to λ values between 1.1 and 1.3.
Since thermal stresses on components of lean-mix engines are lower than those in stoichiometrically-controlled gaseous fuel engines, it becomes possible to significantly increase the boost pressure, so that in combination with charge-air cooling one can achieve effective average pressures of up to 14 bar. The torque band to a large extent corresponds to that of a large number of commercially available liquid-fueled diesel-cycle engines. However, lean-mix engines can still suffer from significant power disadvantages in comparison to the power levels achieved by Euro 3 type liquid-fueled diesel cycle engines.
Since the ability to operate today's lean-mix engines on even leaner mixes is limited, especially in the lower partial load range, to λ values of 1.2 to 1.4, due to the slow rate of combustion of natural gas compared to conventional liquid fuels, these engines also require throttle control. Accordingly, the ECE R49 emission test determines fuel consumption rates that are, depending on the engine design, more than 15% greater than those of comparable liquid-fueled diesel cycle engines. For example, during everyday operation of a city bus, this results in fuel consumption values that are up to 30% higher because of the large proportion of operating time when the engine operates under idle or low load conditions.
Lean-mix concepts for natural gas engines aimed at meeting the new Euro 4 emission standards coming into effect in 2005 are expected be characterized by further developments of existing lean-mix engine concepts aimed at broadening the limits of lean-mix operation to enable reduced NOx emission values below the limit of 3.5 g/kWh.
For this purpose, combustion processes are being developed that are characterized by a more intensive cylinder charging movement, to compensate for the strongly decreasing rate of combustion of very lean mixtures with a relative air/fuel ratio of up to 1.6 at operating conditions close to full load. Lean-mix engines of this type possess combustion processes with increased ability to run on lean mixtures and also are equipped with exhaust turbo-charging and charge-air cooling. Depending on the design, the compression ratio lies between 11.7:1 and 13:1. Such designs should be able to achieve NOx values in the ECE R49 emission test of between 1.5 and 2 g/kWh, given hydrocarbon values upstream of the catalytic converter of approximately 2.9 g/kWh.
Due to the higher compression ratio and the lean mixture under full load, maximum engine efficiency can be increased up to a value of 40%. Consequently, in an ECE R49 test cycle, the fuel consumption values should only be 5% to 15% higher than those of future liquid-fueled diesel cycle engine designs for the Euro 4 emission standard. Depending on the design of the turbo charger, the achievable mean pressure can reach a maximum effective mean pressure of 14 bar to 18 bar.
In addition to developments in the area of homogeneous lean-mixture processes, recent efforts have been directed to processes with high-pressure gaseous fuel injection directly into the combustion chamber of an unthrottled engine. Such engines can employ a compression ratio similar to those employed in liquid-fueled diesel cycle engines because knocking is not a problem. For example with this type of engine, a compression ratio of between 16:1 and 18:1 can be employed. An advantage of this approach is that the low emission levels achievable with a gaseous-fueled engine can be combined with the significantly higher efficiency levels normally only associated with liquid-fueled diesel-cycle engines.
U.S. Pat. No. 5,329,908 discloses a compressed natural gas injection system for gaseous-fueled engines. The fuel injection nozzle is operated so that during the injection process the gaseous fuel spreads as a cloud into the combustion chamber recess through an annular discharge opening being formed during the injection process. During this process, part of the cloud comes into contact with an ignition plug and the fuel-air mixture within the combustion chamber is ignited at the ignition plug. One of the described embodiments uses a constant pressure gas supply and a conventional glow plug serves as the ignition plug. A fuel supply unit is described for ensuring that the gaseous fuel can be supplied to the fuel injection valves with a pressure that is high enough to introduce the fuel into the combustion chamber when the piston is near top dead center. This engine operates in a high efficiency mode that achieves efficiencies like those of a liquid-fueled diesel-cycle engine. However, conventional glow plugs like those used in diesel engines are designed to provide ignition assistance only during start-up because diesel fuel readily auto-ignites at the pressures and temperatures normally present in a diesel engine once it is running. Since gaseous fuels like natural gas do not auto-ignite as readily as diesel, an ignition plug may be needed in the present arrangement to continuously provide ignition assistance to initiate combustion. Continuous activation of a conventional glow plug, which is only designed for brief use during start up, can lead to early failure. Experiments have shown that the length of a glow plug's service life generally decreases as operating temperature increases, and that conventional glow plugs can not be relied upon to provide the durability for continuous activation at the temperatures that operators of gaseous-fueled internal combustion engines are expected to demand.
U.S. Pat. No. 4,721,081 is directed to a glow plug shield with thermal barrier coating and ignition catalyst, which purports to extend the service life of a glow plug that is used to ignite fuels that are not auto-ignitable. In the background discussion provided by the '081 patent, it is noted that it is known to provide protective tubular shields of metal or ceramic circumferentially surrounding a glow plug along its length. The '081 patent further states that it is also known to protect a silicon nitride glow plug by coating the plug with a refractory metal oxide and to provide the glow plug with a combustion promoting catalyst so that the glow plug temperature can be reduced. The improvements added by the '081 patent includes a shield that has an oblique open end that exposes the glow plug in the direction of the fuel injector, while providing a solid physical barrier in the direction of the intake valves. The '081 patent further discloses coating the interior and exterior of the shield with a ceramic refractory material, such as a metal oxide that acts as a thermal barrier so that the shield reduces the cooling effect of the inlet gas on the glow plug and also reduces the electrical power needed by the glow plug to maintain a surface temperature suitable for sustaining good combustion. According to the '081 patent, to further reduce the required glow plug temperature and extend glow plug life, a combustion catalyst can be incorporated into the coating.
There is a need for a gaseous-fueled internal combustion engine that can match the performance, efficiency, reliability, and durability of an equivalent liquid-fueled diesel-cycle engine, while producing lower amounts of harmful emissions such as particulate matter and nitrogen oxides. Such an engine can play a major role in the improvement of air quality, especially in highly populated areas where presently there is concentrated use of liquid-fueled diesel-cycles engines and where gaseous fuels such as natural gas can be easily distributed.